This application claims the priority of German patent document 103 17 501.6, filed Apr. 16, 2005 (PCT International Application No. PCT/EP2004/001826), the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for operating a motor vehicle.
German patent document DE 199 27 975 A1 discloses a method for operating a motor vehicle having an internal combustion engine, an automatic start/stop device for the internal combustion engine and a controllable brake device. When a number of “stop conditions” are fulfilled, the internal combustion engine is stopped by the automatic start/stop device. A “stop condition” here is that the braking force which is applied by a vehicle driver by means of a brake pedal (or the applied braking torque) is sufficient to prevent the motor vehicle from moving. If, by reducing a degree of activation of the brake pedal, the vehicle driver reduces the braking force to such an extent that it would no longer be sufficient to prevent the motor vehicle from moving, the brake device is actuated in such a way that the braking force is maintained. This reduction in the braking force is evaluated simultaneously as a start signal so that the internal combustion engine is started after the brake device is actuated. Actuating the brake device at the end of the stop phase prevents the motor vehicle from rolling away before it is started.
Published U.S. Patent Application 2002/086772 A1 describes a method for operating a motor vehicle in which an internal combustion engine is stopped by means of an automatic start/stop device if the braking force applied by a vehicle driver by means of a brake pedal is greater than a fixed threshold value. If the vehicle driver reduces the braking force to such an extent that it becomes smaller than the fixed threshold value, a brake device is actuated in such a way that the braking force is maintained.
German patent document DE 101 21 158 A1 discloses a method for automatically activating a vehicle brake in conjunction with a starting clutch which is activated by extraneous force. The clutch is activated in such a way that it does not completely interrupt the transmission of torque between a drive motor and transmission. It is checked whether the velocity of the vehicle is zero when a forward gear speed is engaged, or whether the motor vehicle is rolling backwards. If so, the vehicle brake is activated with extraneous force and the starting clutch is opened.
One object of the present invention is to provide a method which achieves low fuel consumption, low exhaust gas emissions, and reliable operation of the motor vehicle.
This and other objects and advantages are achieved by the method according to the invention, in which the braking torque applied by the brake device can be increased by a control device independently of the degree of activation of a brake pedal. The brake device may be embodied as a service brake, parking brake or supplementary brake device of the motor vehicle. The brake device can have a plurality of brake circuits, with it being possible for the driver to apply a braking torque directly by means of the brake pedal via a first brake circuit, and in accordance with a control device via a second brake circuit by actuating suitable actuating elements. The brake device may be activated, for example, electrohydraulically or electromechanically.
According to the invention, at the start of, and during, the automatic stop phase of the internal combustion engine, the control device checks whether the currently acting braking torque is smaller than a threshold value. The control device determines the threshold value as a function of state variables and/or operating variables of the motor vehicle. State variables may be, for example: weight or load of the motor vehicle, state or degree of wear of the brake device. An operating variable may be, for example, a temperature of the brake device. The aforesaid variables may be measured by means of suitable sensors or determined from other variables by means of suitable methods.
When there is a positive result of the check, the control device increases the braking torque to a value which is greater than or equal to the threshold value and maintains this braking torque. The threshold value is dimensioned, for example, in such a way that movement of the motor vehicle is reliably prevented, even when the internal combustion engine is stopped. The first check may occur just before, at the same time as, or just after the internal combustion engine stops.
After the internal combustion engine starts, the braking torque is reduced again, for example via a ramp, in order to start up the motor vehicle. This ensures that the motor vehicle can be started up from a stationary state without rolling in the opposite direction to the desired direction.
With the method according to the invention the motor vehicle is reliably prevented from rolling away during a stop phase of the internal combustion engine. For the internal combustion engine to stop it is thus not necessary for the vehicle driver to apply a braking torque which is sufficient to stop the motor vehicle. The internal combustion engine can be stopped without the risk of the motor vehicle rolling away even if the vehicle driver exerts only a very small braking torque, or no braking torque.
As a result, the internal combustion engine can be stopped frequently, while achieving low fuel consumption and low exhaust gas emissions, in particular in urban traffic. At the same time, reliable operation of the motor vehicle is ensured since the motor vehicle is protected against unintentional rolling away during the stop phases of the internal combustion engine.
When operating variables and/or state variables are taken into account, the threshold value can be defined in such a way that the motor vehicle is prevented from rolling away without the threshold value being set too high. As a result, in addition to reliable operation, a high degree of spontaneity of the motor vehicle is made possible when the internal combustion engine starts automatically.
In one embodiment of the invention, the control device determines the threshold value as a function of environmental variables (such as, for example, a gradient of the underlying surface or an external temperature). These variables may be measured by suitable sensors, or determined from other variables by means of suitable methods. As a result, the threshold value can be defined precisely with respect to safety and spontaneity.
In another embodiment of the invention the braking torque necessary to stop the vehicle is determined by the control device before the brake device is actuated. The aforesaid threshold value is set to a value which is greater than or equal to the specific braking torque, ensuring that the motor vehicle is prevented from rolling away. The threshold value may be higher than the specific torque by an amount constituting a safety margin. As a result, inaccuracies in the determination of the necessary braking torque can be compensated.
In contrast to the method according to the invention, it would always be possible to apply a maximum braking torque during a stop phase. However, since braking torque takes a certain time to be reduced, this would adversely affect the spontaneity of the motor vehicle when the internal combustion engine is started automatically and subsequently accelerated. Furthermore, increasing a braking torque requires an amount of energy which is greater the higher the setting of the braking torque. For example, in a hydraulic brake device it is necessary to build up a hydraulic pressure which increases with the braking torque. By setting only the level of braking torque which is just necessary it is thus possible to save energy (and thus fuel) compared to setting the maximum braking torque.
In one refinement of the invention, during the stop phase the control device monitors whether the motor vehicle is moving, for example, by monitoring rotational wheel speeds or the rotational speed at an output of a transmission which is arranged downstream of the internal combustion engine. For this purpose, the control device can, for example, sense the rotational speeds or receive them from other control devices via signal lines. In the case of movement of the motor vehicle, the control device actuates the brake device in such a way that the braking torque is increased. A movement is detected, for example, if one or more of the aforesaid rotational speeds are higher than threshold values. This process can be repeated during the stop phase so that the braking torque can also be increased repeatedly.
If an excessively low braking torque has been set at the beginning of the stop phase due to uncertainties in the calculation of the necessary braking torque, the motor vehicle is thus reliably prevented from rolling away.
In one embodiment of the invention, the control device increases the braking torque before the internal combustion engine starts. When the internal combustion engine starts, changes may occur in the forces and torques acting on the motor vehicle. Increasing the braking torque ensures that the motor vehicle does not roll even when the conditions change. This is important in particular if the braking torque cannot be set very precisely by the control device.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.